Ok, so in Civil War, Vision is shown to start having feelings towards Scarlet Witch. This goes to such a point in which he "accidentally" shoots down Rhodes. I believe that his love for Scarlet Witch is causing him to lose “connection” with the Infinity Stone and a lack of concentration. In the comics, Thanos says that to be able to wield the Mind Stone, one needs concentration beyond human capacity something that Vision is CLEARLY not doing. Do you guys believe his love for Scarlet Witch could cloud his concentration on the Stone to such a point where Thanos can just go and take it from him easily?